Sion and Nihilus...

I would farm both to be honest. You get about 420 energy a day free. With Sabine and resistance trooper only costing 12 each node thats only 180 for sabine and both resistance trooper nodes. Rey you can farm off the 16 energy node with the falcon, may take a bit longer than two nodes but you can supplement from guild store too. Finn you can also pick a hard node and farm from one node. I believe he's only on a 20 energy so that's another 100. He can also be supplemented from guild store. That is 360 of the 420ish energy you get a day. With a refresh or two you can add 240 to that which is enough to do both sion and nihlas. 100 crystals a day is easily obtainable through dailies and such and if you are decently high in arena even more than that. Gearing and staring nihlas and sion will pay dividends in arena and earn more crystals in the long run making more refreshes possible for other characters in the future. Also no hurry on resistance trooper. Most of the squads he is needed for require jtr so you can always farm him after that if you are short crystals for a refresh or want to farm rey or finn faster. But being as you don't have either vet, you'll have time to farm them off a single node in the time it takes to farm vets. So with that I would use the first 200 energy on sion and nihlas each day and focus gear on them. Then when your arena rank ries you'll have the crystals to do 3 refreshes a day in cantina reducing the time it takes to farm the vets for jtr.